If you'd like to make a few novelty chocolate bars for an event or just because, you can do so without too much trouble by 3D-printing or hand-carving a relief image, making a silicone mold of that image, and pouring melted chocolate into the mold.

Instructables user giovannire came up with this design to make a chocolate bar featuring a Photoshop toolbar. He used a Roland VersaUV which can do some types of 3D bas relief printing. However you get your design together, what you need to do is make a box out of a plastified cardboard, add your mold design, and pour silicone into and form a complete chocolate mold. Silicone mold making rubber usually comes in two parts which you mix together just before using and allowing to cure for 20 minutes or so. After the silicone is cured you can remove it and start making chocolate bars. Admittedly, this is the type of situation where a picture is worth a thousand words, so I encourage you to check out the step-by-step photos and full instructions at the site link below.

We've covered casting custom chocolates with sugar molds which can create 3D chocolates in the shapes of chess pieces and other small items, but the method itself doesn't allow a lot of detail. This seems to be a much more detailed method, but if you don't have access to a 3D printer it might be difficult or time consuming to get it right.